Preparation 2-hydroxy-1-naphthaldehyde cross-linked Fe3O4@chitosan-polyacrylamide nanocomposite for removal of everzol black from aqueous solutions

In this study, new 2-hydroxy-1-naphthaldehyde linked Fe3O4/chitosan-polyacrylamide nanocomposite (Fe3O4@CS@Am@Nph) were prepared. The synthesized nanocomposite was characterized by (FT-IR), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), vibrating Sample Magnetometry (VSM) and Termogravimetric Analysis (TGA). The 2-hydroxy-1-naphthaldehyde modified Fe3O4@CS@Am@Nph nanocomposite was used as an effective adsorbent for removal of everzol black from aqueous solutions by batch adsorption procedure. The effects of important parameters on the surface absorption process of everzol black dye, including pH, contact time, adsorbent dosage and initial dye concentration were studied. The Langmuir, Freundlich and Temkin adsorption models were used to describe adsorption isotherms and constants. The equilibrium results revealed that the adsorption behavior of the everzol black dye on the Fe3O4@CS@Am@Nph nanocomposite fitted well with the Langmuir model. On the basis of the Langmuir analysis, the maximum adsorption capacity (qm) of the Fe3O4@CS@Am@Nph for everzol black was found to be 63.69 mg/g. The kinetic studies indicated that adsorption in all cases to be a pseudo second-order process. Further, the thermodynamic studies showed the adsorption to be a spontaneous and endothermic process.

also be separated from aqueous solutions easily and quickly by using an external magnetic field without requiring tedious filtration or centrifugation 16 . Chemical or physical change of the surface Fe 3 O 4 nanoparticles with some surfactants or polymers is required for improving the adsorption performance of Fe 3 O 4 nanoparticles 17 .polysaccharides such as chitosan and its derivatives are more interesting, since the use of chitosan based adsorbents is one of the best ways to remove the colors and ions of heavy metals even at low concentrations 18 . Chitosan mainly contains poly2-deoxy-d-glucose which is a biopolymer derivative and has well known polymer properties. It has attracted scientist's attentions because of biocompatibility, biodegradability and nontoxic properties [19][20][21][22] . Because chitosan contains high amounts of amine and hydroxyl groups, it has a very high absorption ability to remove many types of metals such as copper, chromium, silver and platinum. However, in order to improve the absorption properties of adsorbents, much attentions have been paid to the design and synthesis of new adsorbents. For example, magnetic chitosan complex coated on the surface Fe 2 O 3 has been used for removing alizarin red from water environments 23 . Wang et al. employed magnetic polydopamine-chitosan nanoparticles as adsorption material for the removal of Methylene blue and Malachite green from aqueous solutions 24 . Zhu et al. synthesized the chitosan-modified magnetic graphitized multi-walled carbon nanotubes for the effective removal of Congo red from aqueous solution 25 . Armagan et al. performed a comprehensive study on the removal of everzol black by Zeolite 26 .
In this study, new 2-hydroxy-1-naphthaldehyde linked Fe 3 O 4 /chitosan-polyacrylamide nanocomposite was synthesized (Fig. 1). The nanocomposite prepared was applied for the removal of the Everzol black from aqueous solution. Moreover, the effects of various parameters such as pH, adsorbent dosage, initial dye concentration and contact time on adsorption behavior were studied. Adsorption isotherms, kinetics and thermodynamic studies have been reported to account for the nature of adsorption process.

Results and discussion
Preparation of Fe 3 O 4 @CS@Am@Nph nanocomposite. In the present study, for the preparation of Fe 3 O 4 @CS@Am@Nph nanocomposite two-step method was successfully used. In the first step, the Fe 3 O 4 @CS@ Am nanoparticles were prepared by reaction of Fe 3 O 4 nanoparticles, chitosan and Potassium persulfate. In the second step, Fe 3 O 4 @CS@Am nanoparticles were connected on the surface of 2-hydroxy-1-naphthaldehyde by the formation of a Schiff base bond between the amine groups of chitosan and the carbonyl group of 2-hydroxy-1-naphthaldehyde. The synthesis route of Fe 3 O 4 @CS@Am@Nph adsorbent are shown in Fig. 1. FT-IR analysis. FT 27 . For chitosan (red line), a broad band around 3425 cm −1 belongs to amino (NH 2 ) and hydroxyl (OH) groups. Beside the peaks at 2916 and 1381 cm −1 assign to C-H and C-N respectively 28 . The FTIR spectra of acrylamide (green line) demonstrated absorption peak at 1674 cm −1 showed the presence of C=O group of amides 29 , also the peaks at 3352, 3192 and 2812 cm −1 attributed to N-H and C-H stretching vibration respectively. The spectrum of Fe 3 O 4 @CS@Am (Fig. 2d) showed broader band at 3442 cm −1 which belonged to O-H stretching vibration. Furthermore, the peaks appearing at 2916 cm −1 and 2879 cm −1 belonged to C-H stretching of the alkyl group. This spectrum also showed that the peaks 1662, 1598 and 565 cm −1 are attributed C=O (amide), N-H and Fe-O bands, respectively. The FT-IR spectrum of the Fe 3 O 4 @CS@Am@Nph (Fig. 2e) showed a peak at 1627 cm −1 resulted from C=N vibration, which can be due to the of the formed Schiff base between the remained free amino groups of chitosan and 2-hydroxy-1-naphthaldehyde. XRD analysis. X-ray diffraction of chitosan, Fe 3 O 4 nanoparticles and Fe 3 O 4 @CS@Am@Nph nanocomposite particles are shown in Fig. 3   displayed three stages of weight loss between 26 and 600 °C. The first stage decomposition occurred between 26 and 230 °C with 10% corresponds to the adsorbed and bound water in the sample 33 . The second stage of weight loss was observed in the temperature ranges of 230-315 °C associated with weight loss 31% is related to the heat decomposition of chitosan structure. And the loss 43% in the range from 315 to 580 °C in the third stage is attributed to the decomposition of cross-linked chains of polyacrylamide. About 16% of the sample retained at 600 °C attributed to the existence of Fe 3 O 4 nanoparticles. Furthermore, the TGA of the Fe 3 O 4 @CS@Am@Nph nanocomposite ( Fig. S.2) showed three stages of weight loss between 26 and 600 °C. The first stage decomposition occurred between 30 and 23 °C with 11% assigned to the adsorbed water in the sample. In two and third stage Brunauere-Emmette-Teller (BET). The BET analysis was used to determine the surface area, pore size, and pore volume of the Fe 3 O 4 @CS@Am@Nph nanocomposite. Figure S.2 represents the BET nitrogen adsorption/desorption isotherm curve of the Fe 3 O 4 @CS@Am@Nph nanocomposite. The surface area, pore volume and pore diameter were found to be 9.47 (m 2 /g), 0.031 (cm 3 /g) and 13.23 nm respectively for Fe 3 O 4 @CS@Am@ Nph nanocomposite. The isotherm curve closely matches to a typical type V isotherm graph confirming the mesoporous property of the nanocomposite 34 .
Magnetization analysis. The magnetic moment of the prepared Fe 3 O 4 @CS@Am@Nph nanocomposite was measured over a range of applied fields between 10,000 and − 10,000 Oe. The magnetization curves of the Fe 3 O 4 , Fe 3 O 4 @CS@Am and Fe 3 O 4 @CS@Am@Nph at room temperature are shown in Fig. 5. The VSM results indicate coating the surface of the magnetite nanoparticles with acrylamide, chitosan and 2-hydroxy-1-naphthaldehyde leads to a decrease in the saturation magnetization. This is due to the presence of acrylamide, chitosan and 2-hydroxy-1-naphthaldehyde on the surface of Fe 3 O 4 nanoparticles which may generate a magnetically dead layer so any crystalline disorder within the surface layer cause to a significant decrease in the saturation magnetization of nanoparticles 35  Sorption studies of selected dyes. Effect of adsorbent dosage. One of the important factors which affects adsorption processes is adsorbent dose since it determines the capacity of adsorbent for a given initial concentration of dye solution 36 . In this study, the influence of adsorbent dose on adsorption removal of everzol    www.nature.com/scientificreports/ Effect of initial pH solution. The pH plays a crucial role in the adsorption of dye onto the adsorbent. Indeed, the pH affects the adsorption process through the degree of ionization, the surface charge of the adsorbent, or the speciation of the adsorbate. In this study, the effect of initial pH on the sorption of everzol black onto Fe 3 O 4 @ CS@Am@Nph nanocomposite were studied at different values from 2 to 12. For this experiment, 0.1 M NaOH and 0.1 M HCl solutions were used to adjust the pH of the solution. The effect of pH on the percentage removal of everzol black by Fe 3 O 4 @CS@Am@Nph is shown in Fig. 6d. In acidic conditions the amount of adsorption is increased that can be due to electrostatic attraction between positive charge of amino groups of chitosan and negative charge of sulfonate groups of the everzol black dye.
Adsorption isotherms. Adsorption isotherm is a method to investigate the relationship between the adsorbed amount in the liquid phase on adsorbent in equilibrium and constant temperature 37 . In fact, the adsorption isotherm describes the interaction between the adsorbent and adsorbed surfaces. Therefore, it is always considered as a fundamental factor for determining the absorbent capacity and optimizing the absorbents 38 . In the present study, Langmuir, Freundlich and Temkin isotherm models were used to obtain the isotherm parameters for adsorption of everzol black onto Fe 3 O 4 @CS@Am@Nph nanocomposite. Investigating the experimental data obtained from adsorption in equilibrium with theoretical models and obtaining the relationship between them provides important information for the best possible design of an absorbent system. Langmuir adsorption isotherm: In this model, there is no interaction among adsorbed molecules and adsorption process happens on homogeneous surfaces, showed in below Eq. (1) 39 : where, C e is the equilibrium concentration of the dye solution (mg/L), q e (mg/g) is the amount of dye adsorbed, q m is the value of monolayer adsorption capacity in Langmuir model and K L : constant value of Langmuir (mg/L). The Langmuir plot for the adsorption of everzol black onto Fe 3 O 4 @CS@Am@Nph nanocomposite at different temperatures is shown in Fig. 7. Freundlich isotherm model (2) is the more for the adsorption of components dissolved in a liquid solution, it is assumed that: First, the adsorption is monolayer and chemical, and second, the energy of the adsorption sites is not the same, i.e. the adsorbent surface is not uniform 40 : K F and n are experimental constants where K F is adsorption capacity at unit concentration (L/mg) and n shows the intensity of adsorption. The 1/n values can be classified as irreversible (1/n = 0), favorable (0 < 1/n < 1) and unfavorable (1/n > 1). Calculation of K F and n in Freundlich model for Fe 3 O 4 @CS@Am@Nph nanocomposite shown in Fig. 8. Also, the separation factor (R L ) was calculated by the following Eq. (3): The values of R L can illustrate the shape of the isotherm to be either unfavorable (R L > 1), linear (R L = 1), favorable (0 < R L < 1) or irreversible (R L = 0). The values of Langmuir and Freundlich parameters and the regression coefficients R 2 of the adsorption of everzol black onto Fe 3 O 4 @CS@Am@Nph are given in Table S.1. According to Table S.1, the value of R L was obtained in the range of 0 < R L < 1, that showed adsorption of the everzol black on Fe 3 O 4 @CS@Am@Nph was favorable. The maximum monolayer adsorption capacity (qm) calculated by Langmuir model was found to be 63.69 and regression coefficient value is 0.9959.

Adsorption kinetics.
In order to determine the type of adsorption kinetics pseudo-first-order 41 and pseudo-second-order 42 kinetics were investigated for the Fe 3 O 4 @CS@Am@Nph nanocomposite. The linear equation of pseudo-first-order and pseudo-second-order kinetic are given by Eqs. (6) and (7), respectively: where q e and q t (mg/g) is the amount of dye adsorbed at equilibrium and at time t, K 1 and K 2 (min −1 ) are the rate constants. Figure 10 shows the absorption kinetics using different models. In the pseudo-first-order model, the values of rate constant k 1 and q e are calculated from the straight line plots of log(q e -q t ) vs time (Fig. 10a). The values of first order rate constant (k 1 ), amount of dye adsorbed at equilibrium (q e ) and coefficient of linear regression (R 2 ) were obtained 0.012 min −1 , 28.8 (mg/g) and 0.9566, respectively. As it is shown in the Fig. 10b pseudo-second-order constants can be calculated from the linear plot between t/qt and time. The values of k 2 , q e and R 2 were obtained 0.0094/min, 33.22 (mg/g) and 0.9918, respectively. The q e value obtained by calculating pseudo second order kinetic is close to the experimental value (49.73), also the pseudo second order model has high regression coefficient (R 2 = 0.9918) than the pseudo first order (R 2 = 0.9566).
Thermodynamic studies. In order to investigate the thermodynamics of adsorption, important parameters such as entropy change (ΔS), enthalpy change (ΔH) and standard Gibbs free energy change (ΔG) on the adsorbent at different temperatures (283, 293 and 308 K) were investigated for surface adsorption of everzol black dye. The values of thermodynamic relations of adsorption were calculated using the following equations:  were determined from slope and intercept of plot Ln K L vs 1/T (Fig. 11). Table S Adsorption mechanism. Figure 12 shows mechanism of adsorption of everzol black on Fe 3 O 4 @CS@Am@ Nph nanocomposite. As seen in Fig. 11, the π-π bond interactions between aromatic rings of dye and 2-hydroxy-1-naphthaldehyde, the electrostatic interactions of negatively charged sulfonate groups of dye and the positively charged protonated amino groups of chitosan and also hydrogen bonding interactions between amine groups and oxygen atom of OH group play important role in adsorption of everzol black on Fe 3 O 4 @CS@Am@Nph nanocomposite.
Reusability studies. The reusing of adsorbent is of great importance as a cost effective process in water treatment. The regeneration ability of Fe 3 O 4 @CS@Am@Nph sample was evaluated by studying adsorption-desorption process in four cycle. Figure 13 shows the percentage removal of dye in 0.1 M HCl solution. As can be seen from Fig. 13, after 4 successive cycles, the dye removal percentage decreased slightly and was still 71%. This suggested that the Fe 3 O 4 @CS@Am@Nph nanocomposite is efficient for everzol black.
Comparison with other reported adsorbents. The result obtained by comparing this adsorbent with other established adsorbents were shown in Table 1. As Table 1 demonstrates, that the Fe 3 O 4 @CS@Am@Nph nanocomposite had an acceptable adsorption capacity for everzol black dye in comparison with other adsor-    in DI water/methanol (100 mL), chitosan (CS) (2 g) and acrylamide (1 g) were added. The mixed solution was ultrasonically dispersed for 30 min. The polymerization reaction of acrylamide was initiated by K 2 S 2 O 8 (0.04 g), and the reaction was allowed to proceed for 12 h at 80 °C under nitrogen atmosphere and mechanical stirring. The resulting solid was magnetically separated, washed with water/methanol several times to remove the unreacted ligands and dried under vacuum. Adsorption experiments. Synthesized nanoparticles were used removal of everzol black dye from aqueous solutions. Various parameters such as initial concentration, contact time, adsorbent dose and pH on adsorption were studied. For performing the experiments, solution of 1000 mg/L of everzol black was prepared in deionized water and diluted to obtain the desired concentrations of dye. Different amounts of nanoparticles, varying from 20 to 100 mg, was suspended in a series of 40 mL dye solution with concentrations varying from 40 to 120 mg/L using 50 mL glass flasks. For suitable times from 10 to 20 min the suspensions were stirred and also the effect of solution pH on dye removal was investigated through adjusting by 0.01 N HCl or NaOH solutions. The nanoparticles adsorbent was separated from aqueous solution by an external magnetic field. The concentration of the everzol black was analyzed by UV-spectrophotometer at λ max 600 nm. The amount of the dye adsorbed onto adsorbent (qe in mg/g) and the percentage of the dyes removed from the solution (R in %) were calculated from the equations:

Preparation of Fe
Amount of dye desorbed Amount of dye adsorbed × 100.